The present invention relates to cameras.
In particular, the present invention relates to structures for automatically operating the shutter of a camera.
In connection with cameras which have through-the-lens light-measuring structure, it is well known that in order to control the exposure time automatically it is necessary to provide some means for storing information corresponding to the light intensity at the object to be photographed inasmuch as this information cannot be received just prior to opening of the shutter. Memory capacitors have been commonly used for this purpose. However, such capacitors are of a certain disadvantage in that the circuits required for transferring the information stored in the capacitor to a switching circuit for controlling the shutter often is complicated and easily results in undesirable errors.
It has already been proposed to provide automatic shutter controls for the above purposes of various types wherein a number of pulses are generated in accordance with the light intensity at the object to be photographed with additional factors such as the magnitude of the exposure aperture also being utilized for controlling the shutter. The particular number of pulses is stored, for example, in a digital memory device such as a binary counter, a reversible counter, or a shift register, and comparison pulses are generated in synchronism with opening of the shuttter so that closing of the shutter takes place at the instant when the number of comparison pulses coincides with the number of stored pulses.
Structures of the above type do indeed have a certain advantage in that they involve smaller errors than in the case of shutter controls involving analog type of storing system utilizing, for example, a well-known memory capacitor. The improvement results from the fact that both the storing of the pulses corresponding to the light intensity and the control of the shutter time are carried out in a digital manner.
However, this latter type of construction also suffers from certain drawbacks. Thus it is essential to use with structure of this type, as presently known, a high frequency oscillator as a pulse oscillator to generate the pulses the number of which corresponds to the light intensity at the object to be photographed, when a range as wide as 1/1000 to 15 sec. is utilized for shutter control, inasmuch as the period of time available for storing the pulses is as short as 0.1 to 0.05 sec., which is the period of time which elapses from the instant of depression of the shutter-releasing plunger up to the springing up of the mirror which terminates the measurement of light. The number of pulses is doubled each time the exposure time of the shutter is prolonged by 1 LV, for example, a single pulse being provided, for example, for 1/1000 sec., two pulses being provided for an exposure time of 1/500 sec., and four pulses being provided for an exposure time of 1/250 sec., and so on. Thus, in this particular type of camera, the exposure time for the shutter is stored for every 1/1000 sec.
However, it is unnecessary to store a value for each 1/1000 sec. of exposure time when the light intensity is relatively high at the object to be photographed, while a relatively fine, precisely determined value must be stored for an exposure time corresponding to a range of relatively low light intensity. Particularly when exposing color film which has an extremely narrow latitude, it is preferable to determine the exposure time in relatively fine, small increments situated even between 1/1000 sec. and 1/500 sec.
In order to meet these latter requirements it is essential that the pulse oscillator have a frequency as high as several MHZ and the corresponding circuitry design is complex and difficult to provide.